The use of magnetic devices to engage, hold, lift, transport, position or carry magnetically attractable articles has been well developed and the particular aspect of this field, which is of interest here, is the class of magnetic holders which utilize permanent magnets and the mechanical displacement of a movable member or rotor to switch the device between its attractive and hence magnetic retentive mode, and its magnetically inactive mode in which an object or article can be released.
In magnetic chucks, for example, devices of this type serve to hold a workpiece by the action of the magnetic force during machining operations. Magnetic manipulators may have a handle enabling a worker to hold the device which can be applied to a magnetic body to be displaced, namely, an iron bar, plate, billet or sheet, for transport and emplacement of the latter. The manipulator can have means for connecting it to a crane, hoist, hook or lever for lifting applications.
Generally speaking, a magnetic carrier of the latter type comprises a fixed permanent magnet body which can be referred to as a stator and a movable permanent magnet body which can be referred to as a rotor and these bodies can be constructed and arranged so that, at the working face of the structure, a magnetic field can be generated in one position of the rotor or the magnetic field can be substantially annulled or canceled in another position of the rotor.
The first position will hereinafter be referred to as the effective position and the second as the ineffective position.
For the most part, the two magnet bodies are equipped with permanent magnets of high coercive force, namely ferrite magnets, and the magnetic masses of the two bodies are equal so that, in the effective position, the magnetic contributions or masses M can be additive (M+M=2M) whereas in the ineffective position the equal magnetic masses M cancel out (M-M=0). The rotor may be angularly displaced through 180.degree. between these operative positions.
Regardless of how the parts have been constructed in the past to achieve these results, it is invariably the intent to minimize any air gap in the magnetic path, to eliminate any loss of magnetic field strength. In the past, moveover, strict adherence to very limited tolerances had to be assured in order to minimize any external field which might remain in the ineffective position of the rotor.
Notwithstanding these efforts, almost invariably in the ineffective position of the magnetic holder, a stray magnetic field or residual field could be found at the effective surface of the device. This was particularly inconvenient for hand-held units where even the least residual field could make it difficult to remove the holder from the transported article.
Furthermore, because precise adjustment was required for complete cancellation of any residual field in the inactive position of the rotor, complex efforts were required at frequent intervals for such adjustment and the fabrication of the device was complicated and made more expensive than was desirable.